Merge #12206: qa: Sync with validationinterface queue in sync_mempools

Summary:
fa1e69e qa: Sync with validationinterface queue in sync_mempools (MarcoFalke)

Pull request description:

Commit e545dedf72bff2bd41c93c93eb576929fce37112 moved `TransactionAddedToMempool` to the background scheduler thread. Thus, adding a transaction to the mempool will no longer add it to the wallet immediately. Functional tests, that `sync_mempools` and then call into wallet rpcs will race against the scheduler thread.

Fix that race by flushing the scheduler queue.

Fixes #12205; Fixes #12171;
References #9584;

Tree-SHA512: 14d99cff9c4756de9fad412f04e6d8e25bb9a0938f24ed8348de79df5b4ee67763dac5214b1a69e77e60787d81ee642976d1482b1b5637edfc4892a238ed22af

Backport Core PR 12206
https://github.com/bitcoin/bitcoin/pull/12206/files

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2951